41 Journal of Failure Analysis and Prevention Volume 6(4) August 2006 Introduction Friction stir welding (FSW) is a solid-state weld- ing process that has received worldwide attention, particularly for joining aluminum alloys. [1,2] There have been numerous attempts to characterize the welds in terms of macrostructure, microstructure, hardness, and residual-stress distribution in connec- tion with the FSW of aluminum alloys such as 2024, [3-6] 7075, [7] 7050, [8] 6061, [9] 6013, [10] 6063, [11] 1050, [12] 1100, [9,13] 1080, and 5083. [14] However, there is apparently no systematic attempt to investigate the natural crack initiation site in an FSW joint. Recent work by Booth and Sinclair [4] identified two forms of failure in a 2024-T351 FSW joint under fatigue loading: failure within the actual weld material (nugget), and failure outside of the actual weld, either in the thermomechanically affected zone (TMAZ) or in the heat-affected zone (HAZ). Failure over the nugget region was linked to discon- tinuities in the material flow pattern at the surface. With no obvious defects being seen, the origins of crack initiation within this region were not clearly identifiable, while the failure in the TMAZ and HAZ was initiated by decohesion of large S-phase particles or by transgranular failure. They suggest that precipitation at interfaces may influence the (Submitted April 3, 2006; in revised form May 23, 2006) Characterization of macrostructure, microstructure, hardness, precipitate distribution, residual stress, and cyclic deformation behavior of 2024-T351 friction stir welded joints has been conducted. In- homogeneous microparameters governing the nonuniform residual stresses and cyclic strength are discussed. The cyclic strength of the weld microregimes is controlled by grain size and distribution of precipitates achieved during the weld process. The comprehensive information of micro- and macromechanics is used to assist in understanding the mechanism that governed the fatigue crack initiation, propagation, and life of the welded joints. Keywords: JFAPBC (2006) 4:41-54 © ASM International DOI: 10.1361/154770206X117559 1547-7029 / $19.00 manufacturing standards, mechanical component, mechanical tests, weld Characterization of 2024-T351 Friction Stir Welding Joints A. Ali, M.W. Brown, C.A. Rodopoulos, and S. Gardiner decohesion strength of the intermetallics at a specific location. However, the question about the locations for crack initiation in an FSW remains unclear. Justification is needed to explain why cracks do not initiate at the hardness minima, as reported in Ref 3; why they can initiate in the finest grain region; and what the role is of macrostructure, micro- structure, hardness, and residual stress on the initiation behavior of fracture. In this paper, the comprehensive characterization of micro- and macromechanics of FSW 2024-T351 was performed based on macrostructure, microstruc- ture, hardness, precipitate distributions, residual stresses, and cyclic deformation behavior. Experimental Procedure The investigation was performed on 13 mm thick plate of 2024-T351 aluminum alloy. The FSW was provided by Airbus UK Ltd. Plates 75 by 60 by 13 mm were welded along their long edge, with the weld direction parallel to the longitudinal (rolling) orientation of the plates. Welds have been characterized in terms of their macrostructure, microstructure, hardness, residual stress, and cyclic deformation behavior. For optical observation purposes, cross sections of the welds were mechanically polished to a 0.25 μm finish and then A. Ali, Department of Mechanical and Manufacturing Engineering, The University of Putra, Serdang, 43400, Selangor, Malaysia. M.W. Brown, Department of Mechanical Engineering, The University of Sheffield, Mappin Street, Sheffield, S1 1JD, United Kingdom. C.A. Rodopoulos, Materials and Engineering Research Institute, Sheffield Hallam University, City Campus, Howard Street, Sheffield S1 1WB, United Kingdom. S. Gardiner, Airbus UK, New Filton House, Bristol BS99 7AR, United Kingdom. Contact e-mail: C.Rodopoulos@shu.ac.uk.